It has been known for more than 30 years that glasses doped with erbium ions can operate as lasers (see, e.g., E. Snitzer & R. F. Woodcock, "Yb.sup.3 +--Er.sup.3 + Glass Laser," Appl. Phys. Lett. 6, 45 (1965)). Early work on erbium glass lasers used silicate glasses and incorporated ytterbium ions as a sensitizer that would absorb pump energy and transfer it to the erbium ions. Within a few years, however, it was shown that phosphate glass was a significantly better host material for this ytterbium sensitized erbium laser system (see, E. Snitzer, R. F. Woodcock & J. Segre, "Phosphate Glass Er.sup.3 + Laser," IEEE J. Quantum Electronics 4, 360, (1968)). Subsequent work with other glasses and crystals demonstrated that, because of its phonon energies, phosphate glass is a uniquely efficient host material for this laser system (see, e.g., V. P. Gapontsev et al., "Erbium Glass Lasers and Their Applications," Opt. Laser Technol., 189 (1982)).
A laser using ytterbium-sensitized erbium-doped phosphate glass as the gain medium can be pumped with different types of pump sources. Ytterbium ions in phosphate glass have a broad absorption peak stretching from 800 nm to 1100 nm, with a peak at 975 nm. Well established pump sources include InGaAs laser diodes generating wavelengths between 940 nm and 990 nm, and neodymium lasers generating wavelengths between 1040 nm and 1080 nm. U.S. Pat. No. 3,582,820 to Snitzer discusses intracavity pumping of an erbium laser with neodymium lasers. End pumping with a neodymium laser has been discussed in detail by D. Hanna, et al., in Optics Commun. 63, 417 (1987). A compact intracavity pumped erbium laser has been described by D. W. Anthon & T. J. Pier, in "Diode Pumped Erbium Glass Lasers," Solid State Lasers III, Gregory J. Quarles, Editor, Proc. SPIE 1627, 8-12 (1992). Pumping with laser diodes in the 940 nm to 990 nm region has been used in a side-pumped configuration by J. A. Hutchinson & T. H. Allik, in "Diode Array Pumped Er,Yb: Phosphate Glass Laser," Appl. Phys. Lett. 60, 1424-6 (1992), and in an end pumped geometry by P. Laporta et al., in "Diode Pumped CW Bulk Er:Yb:Glass Laser," Optics. Lett. 16, 1952 (1991).
Recent interest in erbium glass lasers comes from the desire to produce a suitable laser source for externally modulated CATV transmission systems. In a CATV system, analog optical signals are transmitted through optical fibers over tens of kilometers. Nd:YAG lasers operating at the wavelength of 1318 nm have been shown to be acceptable as light sources for signal transmitters, and much of the experience with the CATV technology has been achieved using these devices. Nevertheless, it is highly desirable to provide a suitable laser source that operates at 1550 nm wavelength. This is because a typical fused silica optical fiber has the lowest attenuation around that wavelength. The low attenuation allows an optical signal at that wavelength to be transmitted over a longer distance. Because the gain of erbium-doped glass covers a range of wavelengths centered around 1550 nm, there is currently strong interest in developing suitable erbium glass lasers for transmitting CATV signals.
There are, however, numerous requirements for a laser source used in the CATV signal transmission system that make it difficult to find a suitable erbium laser for such an application. One difficulty is related to bandwidth and dispersion. For Nd:YAG lasers operating at 1318 nm, where the dispersion minimum of optical fibers lies, a relatively wide (up to 200 Ghz) bandwidth is acceptable. The dispersion at 1550 nm, however, is nearly ten times higher than that at 1318 nm. Even with the dispersion compensation techniques developed in the last few years, a narrower bandwidth would be required at 1550 nm. In most cases, it is more effective to start with a single-mode laser source, and then apply external phase modulation to the laser source to achieve the desired bandwidth.
Single-mode lasers, however, are sensitive to external perturbations, because small cavity length changes can cause the laser to shift to different longitudinal modes. Such mode hops often coincide with periods of two-mode operation that cause unacceptable dispersion effects.
Pressure sensitivity also contributes to the instability of single-mode lasers. The refractive index of air at room temperature is approximately 1+(.DELTA.n P), with .DELTA.n.apprxeq.0.00029 atm.sup.-1. Although this number appears to be small, the changes in the optical length of the cavity resulting from ambient pressure changes can cause severe mode hopping, which significantly degrades the performance of the laser.
Power is another important issue. At 1318 nm, it is possible to extract several hundred mW from a Nd:YAG laser. Even though the power requirement is usually somewhat lower due to the significantly lower losses at 1550 nm, an erbium laser for the CATV application is still required to operate at an output power level about 150 mW. Due to the rather poor thermal and mechanical properties of phosphate glass, operating an erbium laser at such a high power level can produce a significant amount of heat in the erbium glass and cause damages such as fracture or surface melting. Furthermore, heat in the gain medium can also cause severe thermal lensing that significantly degrades the quality and quantity of the laser output.
Besides meeting specific technical requirements such as power and signal/noise ratio, it is also imperative for an erbium laser developed for commercial applications, such as the CATV application, to be robustly constructed. A laser that provides adequate performance on an optical table may not be suitable for commercial uses. For example, the laser may not able to survive through the type of rough handling it is likely to experience during the installation of the optical transmission system. Similarly, the laser may not function properly in an uncontrolled environment characterized by vibrations, pressure changes, and temperature fluctuations. If the laser is not rigidly constructed, the alignment of its optical elements is likely to be lost either due to handling or due to external perturbations in the environment.